Introduction
This list is restricted to those artists who (primarily) work on their own -- sorry, Lennon/McCartney, Elton John, REM, Counting Crows, etc.
It also includes only artists who have released more than two albums. I've listed some potential future additions at the bottom.
I've consciously considered the "singer" part of singer-songwriter -- all of these performers are masters of delivery. Most lack traditionally good voices, but I can't imagine anybody else singing their songs as well. Also in that category, though just missing the list, are Bob Dylan and Elliott Smith.
1. Rufus Wainwright
2. Lucinda Williams
3. Elvis Costello
4. Lyle Lovett
5. Paul Simon
6. Fiona Apple
7. Ben Folds
8. Aimee Mann
9. Ron Sexsmith
10. Stew
Runners-up (alphabetical):
Beck
David Byrne
Bob Dylan
Joe Jackson
Randy Newman
Michael Penn
Elliott Smith
Dar Williams